Frequency control apparatus



Aug. 2, 1966 J. A. MoULToN FREQUENCY CONTROL APPARATUS 3 Sheets-Sheet l Filed March 1, 1963 ATTORNEY Aug. 2, 1966 J. A. MOULTON FREQUENCY CONTROL APPARATUS 5 Sheets-Sheet 2 Filed March 1, 1963 INVENTOR.

JAMES A. MOULTON Nm OPomPmQ 20mm di .Nm .:llll -..lllll n h. o8| i ma om W w Rulli. UIIH O \|l||.. n Nw m |..I..|.|| Ill. IIIIL f 1 1 l I. l \1 N N.\ N f\w F mm ro mojmo l- 1 E 60281 ATTO RN EY Aug 2, 1955 l J. A. MouLToN 3,264,565

FREQUENCY CONTROL APPARATUS Filed March 1, 1963 3 Sheets-Sheet, 3

( IIW l20c! l I FLI-` h 70 0N f (c) l OFF- ON l /l I d A OFF 72 I (e) ON I oFF 7 ON l I3 (f) oFF I l l iO t| t?. t3 t4 '5 TIME, t -v FIG. 4

INVENTOR.

JAMES A. MOULTON BY I. I j

ATTORNEY United States Patent O 3,264,565 FREQUENCY CONTROL APPARATUS James A. Moulton, Santa Ana, Calif., assigner to North American Aviation, Inc. Filed Mar. 1, 1963, Ser. No. 261,980 12 Claims. (Cl. S25-420) The subject invention relates to frequency control apparatus and more particularly to automatic means for controlling the -IF frequency of an electrical system.

In the design of microwave -or radar receivers, it is desirable to reduce the frequency of the received microwave signals from the microwave region to a so-called intermediate frequency (IF) region. Such reduction in frequency of the signal characteristic avoids having to transfer such received signals by means of bulky microwave plumbing, and enables processing of the .signals by conventional electronic .ampliiiers (called IF strips). Hence, by means of such IF amplifiers, the level of the received signals can be raised to provide effective signals for signal utilizing means such as control Idevices and signal display devices. In order to effectively increase the signal levels of such received signals, while minimizing noise levels of signal noise generated within the receiver apparatus, such 'IF strips are conventionally tuned `to a specific ITF frequency, and use a local oscillator in cooperation with a frequency mixer to reduce the received microwave signal to such IF frequency. vIn this way, maximum power trans- -fer and power .amplification occur.

Where, however, the mixer output frequency drifts due to a drift in the frequency of the transmitted (received) pulse or of the local oscillator output or both, then such power amplification is not efficiently effected and the output signal quality is deteriorated by transmission through :such lF strips -or stages. Such frequency drifts may occur due to temperature sensitivity of the transmitter and the receiver local oscillator system. Such drift of the IF signal frequency may be reduced by means for providing automatic control -of such `frequency. However, in the correction of large transient errors in such frequency, it is possible for such frequency control means to attempt to control to a constant IF frequency (fm) determined by either (l) the differences between the frequency (fR) of the received signal and .a higher frequency (LO) of a controlled local oscillator, or (2) the difference between the frequency (fR) of the received signal 4and a lower frequency (fm) of a con-trolled local oscillator. lIn other Hence, ystable operation of an automatic frequency control means could occur at fuir-K, for either However, at -one of such IF frequencies such stable operation may be only neutrally stable, whereby the occurrence -of `a frequency error of a given sense may cause the system to make a change in frequency of the wrong sense -in response to such error. In such event, the magnitude `of the error is increased.

Further, where large transient errors occur in the IF frequency, not only isit .possible for the system to attempt to control to a neutrally stable operating point, but it is also possible for the system to attempt to control to -a higher harmonic of an incorrect lIF frequency, which incorrect IF frequency is lower than the desired IF. In

3,254,565 Patented August 2, 1966 ICC IF signal frequency, whereby the resultant IF frequency represents a sub-harmonic of the desired IF frequency.

The concept of the subject invention provides improved means for generating .a regulated IF frequency.

In a preferred embodiment of the invention there is provided a closed loop frequency con-troller for providing .a constant IF frequency characteristic ina pulsed sinusoidal signal, comp-rising .a voltage-controlled local oscillator, Iand a signal mixer responsive to the local oscillator output and to a pulsed sinusoidal input signal for providing Ian IF signal having a beat frequency indicative of the difference between the respective frequencies of the input signal and lthe local oscillator output.

There is also provided control means, including a phanltas'tron circuit, responsive to the outputof the m-ixer and operatively connecte-d to provide a control voltage input to the local oscillator which is indicative of the difference between the beat frequency of the mixer output and a desired 1F frequency. There is further provided signal gating means for .gating the opera-tion of the pbantastron circuit .as a function of the sense of the output from the frequency discriminator and the state of the phantastron output to establish control of the local oscillator to only one of several possible frequencies which, when combi-ned with the received signal, provide the desired IF frequency, and including means for suppressing system response to harmonics of a resultant 1F frequency.

`In one mode of `operation of the above described arrangement, the initial scanning action or progressive increase of the .phantastron output causes the frequency 0f `the local oscillator to vary or progressively increase, whereby the resultant IF frequency is made to correspondingly scan or vary. The response of the frequency -discriminator to the scanning of the IF frequency is employed to set the signal gating means to a -second mode of operation, whereby ythe phantastron may be made to operate as an operational amplifier in closed loop control of the IF rfrequency only when the frequency of the local oscillator is lat -or near to la selected one of the possible frequencies resulting in the desired IF frequency. A change in this frequency condition o-r in the state of lthe phantastron will reset the gating means, restoring the device ito the first mode of operation.

Accordingly, it is a broad object of the subject invention to provide improved frequency control means.

It is also yan object `of the subject invention to provide means for converting a microwave signal of inconstant frequency to .an IF signal 4of fixed frequency.

It is .another object of the invention to provide improved IF phantastron circuit mea-ns for control of an IF frequency.

'It is stillv another object of the subjec-t invention to provide means for control of an IF frequency to .a selected one of several possible local oscillator frequencies.

It is a further object of the invention to provide means .for `suppressing response of an automatic frequency controller to harmonics of `an undesired IF frequency.

These .and other objects of `the subject invention will become apparent from the following description taken to- .gether with the accompanying drawings in which:

FIG. 1 is .a functional block diagram of preferred embodiment lof the system;

FIG. 2 is a series of response curves spouses of several elements of FIG. l.

FIG. 3 is a schematic diagram of ya preferred embodiment of the bistable logic element of FIG. 1.

FIG. 4 is a time history lof the various signals associated with phantastron circuit of FIG. 1.

illustrating the re- In the figures, like reference characters refer yto Vlike parts.

Referring to FIG. 1, there is 'illustrated a functional block diagram of a preferred embodiment ofthe inven-l y tion." There is provided amicnowave mixer -having an input responsively coupled to a pulsed microwave energy source (not shown), suchas the microwave .transmitter of a pulsed microwave radar system. The frequency ofv such pulsed energy may be 9.3 kilomegacycles per second, for example, although other frequencies could be used.

The function of the microwave mixer is to reduce the frey quency of the pulsedmicnowave enengy 'to a constant IF frequency. This is accomplished by further. responsively connecting the .microwave mixer 10 4to a voltage controlled local oscillator 11, the .output frequency of which is adjusted so as to maintain Vthe difference between the frequency ofthe microwave energy source and .that of" the ilocal oscillator output at a constantvalue, representa ing the desired IF frequency.

The frequency of the out-put from local oscillator 11 is a function of the amplitude of an lapplied analog input, voltage. Therefore, the output frequency from ,the .voltage controlled oscillator 11 Vmay be varied by v.varying the .amplitude of va control voltage applied tothe input;

of oscillatorll.

- The construction andarrangement of oscillator 11 and microwave mixer 10 are well-known to those skilledin the lara-as; is indicated for vexample in U.S. Patent No. 2,933,980, issued Api-i126, 1960 Ito I. R. Moore etal. for an Integrated Aircraft and Fire Control Autopilot. Therefore, these .elements are shown in block form only n FIG. 1.

Control means for regulating the IF frequency of the microwave mixer output-toa constantva-lue is providedl in FIG.1 by the combination comprising a frequency discriminator 12 responsiveily coupled to the IF output of Y mixer 10, and a cyclically scanning phantastron circuit 13 responsively connected to the output of frequency dis-l criminator 12.I The function of frequency discriminatorv =12 ris to provide `a signal output indicative of the. devia-r tion of the pulsed-IF input thereto from a desired IF fre-4 quency -such'as, for example, 30 megacycles per second,

although other frequencies could be specified. The nature of the discriminator response will be described more fully. hereinafter, in connection with FIG. 2. Intenposed between the input to discriminator; 12 land the output of mixer 10 is an IF amplifier 14.4 Amplifier 14 is not essential to the concept of the-device of FIG. 1, and servesonly to assure an `adequate signal level for the input signal applied Ito frequency discriminator 12..

Phantastronr element 13 provides a first output sign-al on line 15 in response to the state ofthe output signal fed fnom frequency vdisc-rimin'ator `12, to the input of element 13. Such output signal on line 15 is fed to the control input: of voltage-controlled oscillator 11. The

nature of the response of phantastron 13 will be describedr more fully hereinafter in connection with FIG. 2. The construction and arrangement off phantastron 13 lis well known tothose skilled in the art; and is described, for example, at page 64 (including Fig. 3.29) of Microwave Receivers by Van Voorhis (volume 23 of the Radiation Laboratory Series) published by McGraw-Hill (1948).r

Accordingly, this element is shown in block yforni only inFIG.1.V

A pulse amplifier 16 and pulse stretcher 17 are interposed between the output from frequency discriminator 12 and Ithe input to phantastron 13. The function of pulse stretcher 17 is to provide a signal input to phantastron 13 which is indicative of the sense and magnitude of the pulsed outputfrom discriminator -12, buthaving .a longer duration. Such longer duration is required to provide a control input to phantast-non to which it is= able to respond. Pulse stretcher 17 mayv be comprised of a well-known circuit element having a fast charge time compared to the discharge time of the required duration,

as will be :described more [fully hereinaften. in lconnec tion with the description of FIG". 23; .The function of pulse.

amplifier 16 is merely Ito provide an adequate signal level of .the output of -frequency ,discriminator i12. The construction'and arrangement of discriminatox` 12,2.amplifier 16randzpulse stretcher 17` lbeing well known to; those vskilled :inthe art, .these elements are shown in block form only.Y

The operation of the system described above and illustrated in FIG. 1 may be more easily understood by reference :to the response .curves shown inzFIGS. .2a and 2b.

Referring to FIG. 2,V there-are illustrated in' FIGS. V2a

and.2b the'responses of phantastron l13a, Iandtdiscriminatorv 12 irespectivelylofFIG. 1.. In'rFIG.' 2a" is illustrated the time history of the phantastron respon-sei .Curve 20 illustrates the cyclical ornperiodic scanningff or lramip [function type output signal which results froma phantast-ron input of a given polarity or sense (say, positive sense), commencing atk time, al1.v Such a response signal demonstrates an amplitude lwhich constantly increasesI in time. until azmaximum negative level-is reached (say, at time.

t4), at which point orconditionwthe phantast-ron operates to recycle, resetting :itself to. the. Ycondi-tion` existing prior to time `t1 .,(wh-ichreset condition is vshown at time .135)

in FIG.4 Uxpcn lreachingsuch lreset conditi-on, the.

phantastron repeatsV the .previous cycle of operation.

Should the senseoflthe phantastroninput signal be.

subsequently reversed` during such scanningV mode .(eg.,

during jthe time interval fromv t1'to.t4 in FIG.V 2d), the; scanning mode will be interrupted (say at time its), and.

the-device will then operate .in the mode or manner.y of a D.-C. operational amplifier. (as indicated by "curve 21), the output thereof fluctuating about a mean value as a direct function; of `thefapplied input. z.

Such modes vof operation. .of a prhantastr-on are :Welllcnown ato those fskilleddnrthe art, asmay be .seen from the above-mentioned volume. 23of the yRadiation `Laboratory'Series.

Referring to FIG. ,2b, there .is illustrated thefrequency 'response of frequency discrirninator 12 of FIG. 1. Ourves 22a and 22b illustrate the amplitude andgsense of the.

D.C.' outputor-response asa function yof the .deviation of the frequency of ythe discriminator IF; input from -a reference frequency, such as, for example, 301 megacycles. As illustrated, curves 22a-.and 22b .show-a null or, zero amplitude output for inputsof"30megacycles, With nega-f tive-sensed outputs for `inputs below 30 megacycles,and positive sense .outputs forr inputs above 30=megacycles.

Such response .cha-racteristicsof, a frequency discriminator are Well known to .those skilled in theY art; However, it ,isito be.. understood that the null or zero amplitude output condition associated ,with the .exemplary 30 ,mega- Y cycle inputmay occur foleither one of Vtwo conditions,

in the `application shown in FIG. 1. A 30 megacycleIF signal outputf-rom mixerfl yWilloccur when the output frequency of local oscillator 11 differs from the frequency of the first input to mixer 10i by 30 megacycles, being either 30 megacycles above or 30 megacycles below the frequency of such first input. For example, if the-frequency of the first inputis nominally 93,00 magacycles, a null can bemade to .occur at the output of the frequency discriminator 12 for a nominal local oscillatorfrequency of either 9270 meg-acycles vor 9330imegacycles. Hence, the, f

in response to the .deviationofz the IF frequency output Should the frequency of of mixer from a desired IF frequency. The sense of the applied input to voltage-controlled oscillator 11 is selected so as to cause the frequency of oscillator 11 to vary in such a sense or direction as to reduce the deviation between the mixer IF frequency output and the desired IF frequency.

Examples of such operation may be considered for the case of frequency tracking or control to an upper side band of the mixer response (e.g., wherein the desired IF frequency is obtained by maintaining the local oscillator frequency 30 megacycles higher than the frequency o-f the first input to mixer 11). For example, if the frequency of the local oscillator output were adjusted to 9330 megacycles, and the frequency of thel first input to mixer 10 should drift from 9300 megacycles up to 9310 megacycles, causing tan IF frequency of (9330-9310) or megacycles or an IF error of -10 megacycles, the resulting sense and amplitude of the input signal to local oscillator 11 would cause the local oscillator frequency to be increased to 9340 megacycles, whereby the desired difference or IF frequency of 30 megacycles is achieved.

Similarly, with a nominal first mixer input frequency of 9300 megacycles, if the frequency of local oscillator 11 should drift from 9330 megacycles down to 9320 megacycles, causing an IF frequency of (9320-9300) or 20 megacycles or an IF error of -10 megacycles, the resulting sense and amplitude of the input `signal to local oscillator 11 would cause the local oscillator frequency to be increased to 9330 megacycles, whereby the desired difference or IF frequency of 30 megacycles is achieved.

In other words, the described automatic frequency control device is arranged to track or seek a desired 1F frequency by causing the local oscillator 11 in FIG. 1 to provide a frequency which is greater than the frequency of the first to mixer 10 by an amount equal to the desired IF frequency.

If, however, a null signal were provided by the output of discriminator 12 due to the frequency of local oscillator 11 lbeing 30 megacycles lower than the frequency of the first input to mixer 10, then an undesirable control condition would exist. Under the condition of a nominal first input of 9300 megacycles to mixer 10, a drift of -l0 megacycles in oscillator 11 from 9270 down to 9260 megacycles will not result in an IF frequency of 20V megacycles. Instead, the resulting IF frequency or difference between the local oscillator output and the first mixer input is (9300-9260) or 40 megacycles. Further, and more significantly, the sense of the output from discriminator 12 is reversed relative to that associated with the 10 megacycle local oscillator drift from 9330 megacycles for the prior described upper side band case, as is illustrated by curves 22a and 22b in FIG. 2b.

In other words, the automatic frequency control system of FIG. 1 will not operate correctly around the lower side band condition where the local oscillator frequency is less than the frequency of the first microwave input to mixer 10, if the control sense of the system response has been designed for operation about its upper side band condition (e. g., local oscillator frequency greater than that of the rst mixer input). Instead, the sense of the control system response is reversed at such lower local oscillator frequency thus preventing proper operation in reducing IF frequency errors.

In normal design practice, it is intended that the local oscillator 11 be allowed to sweep through only a limited range of frequencies, as is represented for example by the LO region in FIG. 2b of 9300 to 9350 megacycles per second. However, the performance of local oscillator 11 to within such a restricted band of frequencies is dicult to maintain, due to transients in the response of the automatic frequency control system of FIG. l and also due to the temperature sensitivity of both the local oscillator and the transmitter (not shown) associated with the radar receiver employing such automatic frequency control system. Further, where the transmitter is tunable (e.g., transmitter frequency is adjustable), tuning adjustments in the transmitter may result in a selected narrow performance range of local oscillator frequencies becoming incorrect. Further, the use of a broad range of local oscillator scanning frequencies would include a lower as well as an upper, local oscillator frequency deviating from the transmitter frequency by the amount of the desired IF frequency. Hence, the likelihood of improper control action arises, as indicated by the exaggerated range of the phantastron scanning action shown in FIG. 2a, resulting in the associated response of FIG. 2b. Accordingly, means are required to allow such broad range of scanning while preventing such improper control action.

Referring again to FIG. 1, there is illustrated means for preventing control about a lower side band of discriminator 12, including means for preventing the local oscillator 11 tracking or being controlled to, a microwave frequency below that of the rst microwave input to mixer 10. Signal gating means 25 is interposed between the output of pulse -amplier 16 and the input to pulse stretcher 17 for gating o-r controlling the transmission of input signals to phantastron 13 in respon-se to control signals applied to a control input 26 of gating means 25. There is further provided bistable signalling means 27 for providing a two-state signal output on line 26.

A rst input line 28 of signalling means 27 is connected to the output of pulse amplier 16 and a reset input signal line 29 of element 27 is connected to phantastron 13.

In normal operation of the above described arrangement, bistable signalling means (reference numeral 27) turns-on gate 25 in response to input signals of a given sense, say negative sense, on input line 28. Hence, line 28 in feeding a negative signal input to bistable means 27, serves as enabling means to enable gate 25 to be closed, thereby allowing closed-loop operation of the automatic frequency control system of FIG. l. In other words, the output state of bistable signalling means may' be changed from the OFF state to the ON state by the application of an input signal of negative sense on line 28.

If, however, a positive polarityk input signal is applied to line 28, bistable signalling means 27 would remain in the OFF state. Such positive input might correspond, for example, to the output of frequency discriminator 12 (in cooperation with pulse amplifier 16) associated with a local oscillator frequency below 9270 megacycles (as illustrated by curve 22a in FIG. 2b) in response to the scanning action of phantastron 13 (as illustrated by curve 20 for the time interval between t1 and t2' in FIG. 2a). As the scanning action of phantastron 13 continues (curve 20 between t2 and t3 of FIG. 2a), the sense of the output from frequency discriminator 12 changes from a positive sense and provides a negative sense (curves 22a and 22b between local oscillator frequencies 9270 megacycles and 9330 megacycles), which negative signal providesv an enabling signal input to bistable signalling means 27. Bistable means 27 responds to the enabling signal on line 28, turning on gate 25. Such closing of gate 25 does not interrupt' the sweep mode of phantastron 13 but enables the subsequent operational amplifier mode of phantastron 13 to close the automatic frequency control loop, as is explained. more fully hereinafter.

In the region of t3 of the phantastron response (curve 20 of FIG. 2a, corresponding to the null (zero amplitude) region of the discriminator response curve 22b in FIG. 2b, associated with a local oscillator frequency of about 9330 megacycles),` the phantastron will operate as an operational amplifier, whereby the frequency control loop functions as a closed loop proportional controller. If, however, the frequency of local oscillator 11 of FIG. l were to increase slightly above 9330 megacycles corresponding to an increase in IF frequency output of mixer 10- toslightly above 30 megacycles, the corresponding a change in -state of the output kof bistable signalling means .27. If, on the other hand, 'the IFfrequency error resulting from an increaseinthe IF frequency above the desired 30 megacycles should become so great,

as to causephantastron 13 kto recycle (corresponding to curve of FIG. 2b between t4 and f5), then a signal indicative of such ,condition is fed from phantastron` 13 on line 29 to reset bistable means 27, causing a change of state of the two-state sgnalon linef26, whereby gate is switchedoi Such large frequency error or transient might occur, for example',.where the radar transmitter ymissesfa pulse and no received irst'input occurs at mixer 10.V In suchevent of phantastron 13 recycling through the scanning mode, bistable signalling means 27 and signal gating means 25 would cooperate as before, whereby the system of FIGfl is prevented from lirnproperly controlling to thenull condition of frequency discriminatorylZ, corresponding toa local oscillator frequency of 9270 megacycles in FIG. 2b.

Y Hence, it is to be appreciated that the cooperation-of gating means 25"and bistable means 27 with phantastron 13 prevents the system from controlling to a lower side1 band` IF frequency produced by maintaining theatrequency of local oscillator 11 below that of the first microwave input to mixer 10.

If, howeverythe output response of microwave mixer 10 in FIG. 1 contains harmonics of the difference fre-` quency or` IF outputs, .it is yet possible for the closed loop .control mode vto cause local oscillator 11 to track in response to an upper side band IF frequency of less thanthe desired :value and having aharmonic ofthe desired frequency.l For example, local oscillator 11might be controlledrto a frequency (say, 9315 megacycles) which is above that of the first microwave input `mixer 10 (say, 9300 `megacycles), thereby produ-cinga low frequency IF of 15 megacyclesl andhaving ya rst har- .'nonic of 30 megacycles. The response of discriminator 12 to suchharmonicwould provide-a control signal :ausing the system t-o null to suchharmonic, whereby t local oscillator`11 would be caused to track or follow a. microwave frequency (say of 9315 megacycles),rand maintaining a fundamental frequency of only 15 mega- :ycles at the output of mixer 10. In other words, control ls maintained to a frequency representing a sub-harmonic )f the desired IF frequency. Accordingly, means is re- ;uired to suppress or prevent such adverse response :haracteristic.

Referring again to FIG. il, there is further illustrated neans for preventing control toa frequency represent-` ng a sub-harmonic of a desired IF frequency. There s provided a signal source responsive to a sub-harmonic 3f the desired IF frequency for providing a second reset signal on line y to bistable signalling means 27. Such second sourceof a reset signal comprises lter means l1 (such as a band pass lter or low pass lter) respon-l iively connected to the output of mixer 10 for trans 52 are well known in the art, and therefore such elements rre illustratedin FIG. 1 in block forni only.

By means of the arrangement of filter 31 and detector 52 illustrated in FIG. 1, bistablel means 27 will turnoff gate 25 `in response to reset signals on liney 30, indicating he presence of low frequency signals havingharmonics` S frequencies- Ywhich produce IF frequencies representing sub-harmonics offa desired IF frequency..

A preferred embodiment of fthe` bistabler signalling Y means of FIG. l is shown in FIG. Y3,'they operation 'of which maybe more easily explained yby reference yto Ythe time histories thereof illustrated in FIG. 4.

FIG.` 3 illustrates a schematic .diagram of apreferred embodiment :for bistable means1g27 and siginal gating; means 25,shown in `cooperation-With.anexamplary pulse Vamplifier 16,'pulsestretcher 17sand1-phantastron 13 ofethe AFC system of FIG.Y 1. Theillustrated:interconnection of two transistors 41-V and 42 form a bistablelmultivibrator circuit 27`witl1 current supplied fromy a source of y+15() v. D.C. through resistors 44 and 45.1 However, a portion of --the currentzthrough resistor 44-ilo`ws through resistorf144;LV1

to the screen grid of the phantastrontube 60; At the end of eachlinear p'hantastron Vsweep of phantastron 13,:'re-

cycle occurs .and the screen currenty of vacuum tube `60' increased.

During therecycle period of phantastron 13 (period to to t1 in FIG. 4)i,'the :screen voltageof tubef60 dropsfrom volts to 2.80 volts `(e.g., drops from l'+120 volts to +20 volts with respect to its cathode),` causing the voltage at `the collector ofk transistor 41'of multivibrator 1 26|'to dropwto Vzero or slightly therebelow. Therefore,

comesnon-conductive because no=base current is supplied through resistor 46.1

When transistor 42 is open or non-conductive, current supplied throughresistor45Jfrom the +150`volt'D.V-C.

source `flows through resistor 47fto the baseof transistor 41` andlthrough resistor 49 to the baseof ,transistor .50.; The `current in the base of transistor 50 lcauses collectoremitter junctiony saturation ofk transistor 50 which elfectivelyvshuntsft-o ground any positive pulse (from amplier 16):` attempting `to appear at theljunctions66 of capacitors NegativeV going pulses `at^junction 66 are v SS'iand 54. i shunted to ground through diode 51. Therefore, pulse outputsfrom inverter-amplifier y16 rare not applied Ito the 'pulse-stretcher network'comprisin'g diode 55,1resistorV 56' andcapacitor 54 (curve 73 from to to t1 inFIGIA). In other words,` during recyclingofphantastron 13, signal gate 25 is fropen? or oft4 (curve73' from t0 toltl'in FIG.

4) and the phantastron 13.is unalfectedby either, negativ'ev orrpositivey pulses appearingV aty the pla-te :of tube 36` 1 (offarnplilery 16').

As the phantastron tube i60-starts the next linear sweep (curve 20 at t1. in FIG..!4),ritsscreen current decreases` and the associated screen voltage rises to approximatelyY -180 voltsV (+120 with respect vto the cathode). The

screen voltage of phantastron tube 60. will'remain `at approximately. -180 volts during the sweep and also when thefphantastron is performingas a D..C. amplifier. Atl

this time, positive current is available at the junction :67

lof resistors 44 and y144. i Howevensince a base current :is

present in transistorl, thecollector-emitter junction of transistor 41 is saturated` (curve 70 'at t1 in FIG. 4), due

and 72, respectively betweentl andtz `in FIG. 4), which condition defines a.iirst.stable,state., This stable state vwill continue (even if thephantastron recycles) untila negative `pulse from `amplifier 16 (curve 22`from t2 Vto 133v inFIG'. 54) is appliedrot the` base of :transistor 41.

- After thev .phantastron sweep has caused the -IF -frequency to -pass through a negative discriminator output (as seen at the output. of inverter-amplifier 16 land as1illustrated in FIG. 2b), the logic circuit will allow subsequent f positive pulses from ampliiierf16 (curve 22 between t3 and` t4in FIG.' 4) to tbe appliedto the pulse-stretcher network 17, so thatv microwave frequency tracking or IF` lock-Ion,

may be effected :by the system-of FIG.' 1. `This isfaccomplished by the coupling of negative going .pulses from the plate of tube 36 through capacitor 38 and `diode 48 to the base of transistor 41. Once transistor 41 is cut off, current flows from the junction `67 of resistors 44 and 144 through resistor 46 to the base -of transistor 42, causing transistor 42 to saturate (curve 71 at t2 in FIG. 4). The saturation of transistor 42 thus removes the current formerly applied to' the base of each of transistors 41 and 50, establishing a second stable state with transistor 42 on (curve 71 at t2 in FIG. 4) and transistors 41 and 50 (curves 70 and 72 respectively at 22)' oi As the phantastron sweep continues (curve at yt3 in FIG. 4), positive pulses (curve 22 at t3) will appear at the plate of pulse amplifier 16 when the IF frequency input to frequency discriminator 12 of FIG. 1 sweeps above 30 mc. These positive pulses are now coupled through capacitors 53 and 54 (curve 73 between t3 and t4) Where the positive peak is clamped to -300 by diode `55. Capacitors 53 and 54 then discharge through resistor 56, developing a stretched pulse of negative lpolarity (through resistor 57) at the grip of phantastron tube 60. Inl response to such negative pulse input, tube 60 stops sweeping and acts as `a D.-C. amplifier (curve 20a in FIG. 4).

During the above-described off condition of transistor 50 (curve 72 between t2 and t4), diode 51 has no effect on the pulse amplitude appearing at the junction 66 of capacitors 53 and 54. Instead diode 51 merely clamps the signal to ground, establishing the positive signal swing appearing at the collector of transistor 50 at the pulse amplitude on the plate of amplifier tube 68.

The detail functions of several other elements of the circuit of FIG. 3 are further described. The function of resistor 52 is to allow a negative pulse to be developed at the plate of tube 36 when transistor 50 is saturated. In other words, without the employment of resistor 52, no pulse could occur at the plate of tube 36 when transistor '50 is saturated, because such plate would be shorted to ground, there-by preventing switching of the signal state of gate 25. Resistors 39 and 40 establish a back bias across -diode`48 that prevents system transients present in the 150 v. D.-C. source or which are coupled from the heater to cathode in tube 36 or in discriminator 12 (of FIG. 1) from erroneously cutting off transistor 42.

Capacitor 61 smooths the screen grid waveform (from tube 60) applied to transistor 41 through resistor 144 so that capacitive unbalance in multivibrator 27 will not cause the multivibrator output to assume vthe wrong signal state at the end of the phantastron recycle period. Capacitor 61 also attenuates transients occurring on the -300 v. D.C. supply so that they do not erroneously switch multivibrator 27.

In order to eliminate IF lock-on at 15 mc., a 15 mc. reject pulse is required by the logic circuit of FIG. 3, as explained hereinabove in connection with the description of FIG. 1.

The output of negative detector 32 (of FIG. l) on line 30 is coupled through resistor 43 to the base of transistor 42. In the event of attempted IF lock-on at 15 mc., the resultant negative pulse on line 3Q will cut transistor 42 off In such event, current is then supplied through resistor 45 to transistors 41 and 50, establishing a stable state of multivibrator 27, which results in signal gate shunting a signal pulse at terminal 66. Therefore, the phantastron will sweep until the IF output of amplifier 16 passes through. the desired range of frequencies producing the proper negative-positive discriminator response sequence enabling AFC IF lock-on at rnc.

Means has been described and illustrated for preventing frequency control to an undesired sideband frequency or undesired harmonic of a -desired frequency in an automatic frequency controller. Accordingly, it is to be seen that the device of this invention provides improved and novel means for regulating and controlling the signal frequency of a periodic signal source.

` Although .the invention has been described and illustrated in detail', it is to be clearly understood that the same is by way of illustration and example only, and is not to be taken by way of limitation, the spirit and scope of this invention being limited only by the terms of the appended claims.

I claim:

1. A closed loop automatic frequency controller for providing a constant IF frequency characteristic in a pulsed sinusoidal signal comprising a voltage controlled local oscillator for generating a signal having a frequency indicative of a control voltage,

a signal m-ixer responsive to a sinusoidal input signal and an output of said local oscillator for providing a signal having a beat frequency indicative of the difference between the respective frequencies of said input signal and said local oscillator output,

control circuit means including a phantastron circuit and responsive to said output of said mixer and operatively connected to provide a control voltage input to said local osc-illator which is indicative of the difference between the beat frequency 0f said mixer output and a reference frequency, whereby the frequency of said local oscillator tends -to vary so as to maintain a constant IF frequency at the output of said mixer,

bistable gating means disconnectably intercoupling said output of said discriminat-or and an input of said control means for restricting control of said local oscillator in response to the phase-sense of the low frequency response only of said frequency discriminator and in further response to an abnormally high negative level of the output of said phantastron circuit.

2. The device of claim 1 including means for suppressing frequency controller response to a harmonic of an IF mixer frequency thereby avoiding generation of an IF frequency indicative of a sub-harmonic of the reference IF frequency.

3. The device of claim 1 further including signal detector means responsive to a sub-harmonic of said reference IF frequency,

said gating means being further responsively connected to said signal detector means for suppressing operation of said control means in response to said subharmonic.

l 4. The device of claim 1 including gating means for functionally disabling said automatic frequency controller, second means for distinguishing said an upper side band response of said frequency discriminator from a lower side band response of the same,

third means for detecting an IF signal having a harmonic of like frequency as a reference IF frequency,

said gating means being responsively connected to said sec-ond means for restricting control of said frequency controller to a selected one of said side band responses of said frequency discriminator,

l said gating means being further responsively connected to said third means for suppressing control of said -automatic frequency controller to said harmonic.

5. In an automatic IF frequency controller having a frequency discriminator responsively connected to a signal frequency mixer in cooperation with a phantastron circuit for control of the voltage-controlled local oscillator of said automatic IF frequency controller, means for restricting control of said local oscillator response to the higher one lof the discriminator side band responses capable of being caused by said local oscillator comprising bistable circuit means having a two-state output, said bistable circuit means being responsive to the phasesense of the response of said frequency discriminator for establishing a first one of said tWo-states,said` bistable circuit means being further responsive -to said phantastron circuit for switching to a second one of said two states, and

signal gating means interposed between the `output of said Vfrequency discriminator and the input to said phantastron circuit; and responsively connected to the output of s-aid bistable -circuit means.

6.l In an automatic 1F frequency controller havingy a frequency discriminator responsively connected to a signal frequency mixer in cooperation with a phantastron circuit for control of thevoltage-controlled local oscillator of saidautom-atic 1F frequency controller, means forA restricting control of said local oscillator response to onlyV one of the discriminator side band responses capable of being caused by said local oscillator comprising bistablefcircuit means having a two-state output, said bistable circuit means being responsive t the phasesense of the low frequency response only of said fre'- quency discriminator for providing a first one of said two states, said bistable circuit means being'further responsive to said phantastron circuit for providing a second one of said two states, and

signal gating means for connecting and disconnecting the outputof said frequency jdiscriminator to the in-y put of said phantastron circuit in response to said first and second state respectively of said-bistable circuit means.

7.l The device of'claim 6 Afurther including means for suppressing frequency controller response-to a sub-harmonic of a reference frequency.

8. ,The device of claim 6 furtherincludiug low.frequency band pass means for detecting an IF signal having a `harmonic of like frequency as a reference IF frequency, and

said fbistable circuit means being responsively con-y nected to said-band pass lmeans for providing said second state.

Y 9. The device of claim 6. wherein said bistable' circuit i,

an out-put terminal connected to said second transistor,

i unidirectional .conducting means interconnecing saidy first input terminal to the base electrode of Vsaid first transistor for providing only'. a first signal vstate on Y saidV output terminal in response to only inputs of a preselected sense applied to said firstv :input ter-- minal,. i first impedance `coupling means interconnectmg said second input terminal and one of the collector and emitter electrodes of said first transistor z'for providing a second signal state :on said output terminal in response to inputs of a preselected sense applied f:

to said second input terminal, and

second impedance coupling means interconnecting said a secondy one of the collector .and emitter electrodesY of each Aof said transistors being commonly con nected to a direct current source of potential, a series resistor interposed in series circuit with each said second electrodes,

a rst coupling resistor interconnecting the base elec,

trode of saidfirst` transistor and said .secondv elec.

trode .of said 'second-transistor,

a second coupling lresistor interconnecing the base elec.

trode of said second transistor andsaid secondelec'-A Y first impedance coupling vmeans interconnecting saidI second-inputiterminal and said :second elec-trode of said first transistor, second impedance coupling ,meansr interconnect-ingV said third input 4terminal and said base lelectrodeof said second transistor, t saidfirst and secondin-put:terminalsbeing. connected tothe 4outputs of said frequency discriminatorl andsaid :phantastron:respectively,r andsaid output rterf minalbeing connected to a control :electrode of' saidf signals gating means;

11.i The device of claim 6 lwherein the combination ofsaid .bistable circ-uitmeanseand said signal; gatingmeans comprises first, second and .third :input:v terminals, an output terminal,

a common input-output terminal,

first, secondhand third Ltransistors,.

a first one of the :collector andlemitter electrodes ofeach Vof said Itransistors beingcommonly connected'r to said Icommon terminal, a second one :of the collector 'and emitter electrodes of reach of :said first :and :second transistors being commonlyconnected to. a direct current source :of potential,

a series` resistor linterposed in series circuitwith each:

ing said first: input7 terminal and said secondy electrodei of said third= transistor, p

a first coupling resistor:interconnecting',thebase electrode of said first .transistor and said vsecond electrode of saidy second transistor,`

a secondy coupling resistor' yinterconnecting .the base electrodefoffsaidsecond transistor Iand said second electrode ofesaid first transistor, t

a third coupling resistor interconnecting the base electrode of saidt-hirdztransistor and said second electrode of said secondV transistorgf,

said outputf'termnal ,being :connected tosaid second electrode of said third transistor,

first zunidirectional conducting means' interconnecting said first inputterminal andthe base` elec-trode .of said first transistor, i

second unidirectional conducting meansV connected across said first andsecond electrodes-of saidthird transistor,

first impedance couplingmeansrinterconnecting `said second input terminal and said second .electrode ofV said first transistor, second impedance f coupling ymeans interconnecting` said third input yterminal and said base .electrode vof said lsecond transistor,y

said rst andsecond input terminals beingconnected to the outputs of said frequency `discriminator and `said phantastron respectively, said )third input` terminal being adapted-to be connected .to a signalsource:

for gating said third transistor, yand said :output terminal .beinglconnectedI to an input of :said `phany tastron.

12. A frequency control device comprising` a frequency discriminator; bistablev .means for gating the outputY of 2 the frequency discriminator,v said bistable vmeans having` a first input responsive to ra ,preselected polarity ofy output f v ysignals of said discriininator togate-on said gated ydis- .13 criminator output; and means for providing a varying signal in response to the gated output of said frequency discriminator, a second input of said gating means being responsive to a preselected signal level of said varying signal to gate-off said gated discriminator output; and a mixer, said frequency discriminator connected to be responsive to the output of said mixer; a variable oscillator connected to .provide an input to said mixer and further connected to be responsive to the output of said means providing a variable signal; and band pass lter means responsive to the output signals .provided to said mixer; and means operable in response to said band pass lter to cause said gating means to not conduct whereby a selected frequency is not provided by said mixer.

References Cited by the Examiner UNITED STATES PATENTS 2,852,669 9/1958 Ashby 325-423 X 2,897,449 7/1959 Larkin 331-4 3,174,105 3/1965 Morgan 325--420 10 KATHLEEN H. CLAFFY, Primary Exminer.

ROBERT H. ROSE, Examiner.

R. LINN, Assistant Examiner. 

1. A CLOSED LOOP AUTOMATIC FREQUENCY CONTROLLER FOR PROVIDING A CONSTANT IF FREQUENCY CHARACTERISTIC IN A PULSED SINUSOIDAL SIGNAL COMPRISING A VOLTAGE CONTROLLED LOCAL OSCILLATOR FOR GENERATING A SIGNAL HAVING A FREQUENCY INDICATIVE OF A CONTROL VOLTAGE, A SIGNAL MIXER RESPONSIVE TO A SINUSOIDAL INPUT SIGNAL AND AN OUTPUT OF SAID LOCAL OSCILLATOR FOR PROVIDING A SIGNAL HAVING A BEAT FREQUENCY INDICATIVE OF THE DIFFERENCE BETWEEN THE RESPECTIVE FREQUENCIES OF SAID INPUT SIGNAL AND SAID LOCAL OSCILLATOR OUTPUT, CONTROL CIRCUIT MEANS INCLUDING A PHANTASTRON CIRCUIT AND RESPONSIVE TO SAID OUTPUT OF SAID MIXER AND OPERATIVELY CONNECTED TO PROVIDE A CONTROL VOLTAGE INPUT TO SAID LOCAL OSCILLATOR WHICH IS INDICATIVE OF THE DIFFERENCE BETWEEN THE BEAT FREQUENCY OF SAID MIXER OUTPUT AND A REFERENCE FREQUENCY, WHEREBY THE FREQUENCY OF SAID LOCAL OSCILLATOR TENDS TO VARY SO AS TO MAINTAIN A CONSTANT IF FREQUENCY AT THE OUTPUT OF SAID MIXER, BISTABLE GATING MEANS DISCONNECTABLY INTERCOUPLING SAID OUTPUT OF SAID DISCRIMINATOR AND AN INPUT OF SAID CONTROL MEANS FOR RESTRICTING CONTROL OF SAID LOCAL OSCILLATOR IN RESPONSE TO THE PHASE-SENSE OF THE LOW FREQUENCY RESPONSE ONLY OF SAID FREQUENCY DISCRIMINATOR AND IN FURTHER RESPONSE TO AN ABNORMALLY HIGH NEGATIVE LEVEL OF THE OUTPUT OF SAID PHANTASTRON CIRCUIT. 